1 Interactions of organosulfates with water vapor under sub- and supersaturated
2 conditions
3
4 Chao Peng,1,2 Patricia N. Razafindrambinina,3 Kotiba A. Malek,4 Lanxiadi Chen,1,2,7 Weigang
5 Wang,5 Ru-Jin Huang,6 Yuqing Zhang,1,2 Xiang Ding,1,2 Maofa Ge,5 Xinming Wang,1,2 Akua A.
6 Asa-Awuku,3,4 and Mingjin Tang1,2,7,*
7
8 1 State Key Laboratory of Organic Geochemistry, Guangdong Key Laboratory of Environmental
9 Protection and Resources Utilization, and Guangdong-Hong Kong-Macao Joint Laboratory for
10 Environmental Pollution and Control, Guangzhou Institute of Geochemistry, Chinese Academy of
11 Sciences, Guangzhou 510640, China
12 2 CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
13 3 Department of Chemistry and Biochemistry, College of Computer, Mathematical and Natural
14 Sciences, University of Maryland, College Park, MD 20742, USA
15 4 Department of Chemical and Biomolecular Engineering, A. James Clark School of Engineering,
16 University of Maryland, College Park, MD 20742, USA
17 5 State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National
18 Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular
19 Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
20 6 Key Laboratory of Aerosol Chemistry and Physics, State Key Laboratory of Loess and
21 Quaternary Geology, Institute of Earth and Environment, Chinese Academy of Sciences, Xi'an
22 710061, China
1 23 7 University of Chinese Academy of Sciences, Beijing 100049, China
24
25 *Correspondence: Mingjin Tang ([email protected])
26
27
2 28 Abstract
29 Organosulfates (OS) are important constituents of secondary organic aerosols, but their
30 hygroscopic properties and cloud condensation nucleation (CCN) activities have not been well
31 understood. In this work we employed three complementary techniques to characterize interactions
32 of several OS with water vapor under sub- and supersaturated conditions. A vapor sorption
33 analyzer was used to measure mass changes of OS samples with RH (0-90%); among the 11
34 organosulfates examined, only sodium methyl sulfate (methyl-OS), sodium ethyl sulfate (ethyl-
35 OS), sodium octyl sulfate (octyl-OS) and potassium hydroxyacetone sulfate were found to
36 deliquesce as RH increased, and their mass growth factors at 90% RH were determined to be
37 3.65±0.06, 3.58±0.02, 1.59±0.01 and 2.20±0.03. Hygroscopic growth of methyl-, ethyl- and octyl-
38 OS aerosols was also studied using a humidity tandem differential mobility analyzer (H-TDMA);
39 continuous hygroscopic growth was observed, and their growth factors at 90% RH were
40 determined to be 1.83±0.03, 1.79±0.02 and 1.21±0.02. We further investigated CCN activities of
41 methyl-, ethyl- and octyl-OS aerosols, and their single hygroscopicity parameters (κccn) were
42 determined to be 0.459±0.021, 0.397±0.010 and 0.206±0.008. For methyl- and ethyl-OS aerosols,
43 κccn values agree reasonably well with those derived from H-TDMA measurements (κgf) with
44 relative differences being <25%, whereas κccn was found to be ~2.4 times larger than κgf for octyl-
45 OS, likely due to both solubility limit and surface tension reduction.
46
3 47 1 Introduction
48 Secondary organic aerosol (SOA) contributes approximately 70% to the global atmospheric
49 organic aerosols (Hallquist et al., 2009; Jimenez et al., 2009). SOA can affect the Earth’s radiative
50 forcing and climate directly by scattering and absorbing solar and terrestrial radiation, and also
51 indirectly by acting as cloud condensation nuclei (CCN) or ice nucleating particles (Moise et al.,
52 2015; Shrivastava et al., 2017). Consequently, it is important to understand the source, formation,
53 and physicochemical properties of SOA (Pöschl, 2005; Jimenez et al., 2009; Noziere et al., 2015;
54 Peng et al., 2020). However, SOA concentrations on the global scale are significantly
55 underestimated by many modeling studies (Heald et al., 2005; Kanakidou et al., 2005; Ervens et
56 al., 2011; McNeill et al., 2012; Shrivastava et al., 2017), indicating that there might exist unknown
57 while important precursors and/or formation mechanisms of SOA.
58 Organosulfates (OS), which could contribute to the total mass of ambient organic aerosols by
59 as much as 30%, may largely explain the discrepancy between observed and modeled global SOA
60 budgets (Surratt et al., 2008; Tolocka and Turpin, 2012; Liao et al., 2015). A number of field
61 measurements have observed significant amounts of OS in ambient aerosols in different regions
62 over the globe (Froyd et al., 2010; Kristensen and Glasius, 2011; He et al., 2014; Hettiyadura et
63 al., 2015; Riva et al., 2019; Wang et al., 2019a; Wang et al., 2019b; Zhang et al., 2019;
64 Bruggemann et al., 2020; Wang et al., 2020). For example, the mass concentration of sodium
65 methyl sulfate, the smallest organosulfate, was found to be 0.2-9.3 ng m-3 in Centreville, Alabama
66 (Hettiyadura et al., 2017). Hydroxyacetone sulfate, which may originate from both biogenic
67 (Surratt et al., 2008) and anthropogenic emissions (Hansen et al., 2014), has been detected at
68 various locations, such as the Arctic (1.27-9.56 ng m-3) (Hansen et al., 2014), Beijing (0.5-7.5 ng
69 m-3) (Wang et al., 2018), Shanghai (1.8-2.3 ng m-3) (Wang et al., 2021), Xi’an (0.9-2.6 ng m-3)
4 70 (Huang et al., 2018), Centreville (1.5-14.3 ng m-3) (Hettiyadura et al., 2017) and Iowa City (4.8±1.1
71 ng m-3) (Hughes and Stone, 2019). In addition, benzyl and phenyl sulfates were also ubiquitous in
72 the troposphere, with reported concentrations up to almost 1 μg m-3 (Kundu et al., 2013; Ma et al.,
73 2014; Staudt et al., 2014; Huang et al., 2018).
74 As OS are ubiquitous in the troposphere, it is important to understand their hygroscopic
75 properties and CCN activities in order to assess their environmental and climatic effects
76 (Kanakidou et al., 2005; Moise et al., 2015; Tang et al., 2016; Tang et al., 2019a), especially
77 considering that OS could contribute up to 30% of total mass of organic aerosols in the troposphere
78 (Surratt et al., 2008; Tolocka and Turpin, 2012; Liao et al., 2015). However, to our knowledge,
79 only two previous studies have explored their hygroscopic properties and CCN activities (Hansen
80 et al., 2015; Estillore et al., 2016). Hansen et al. (2015) investigated hygroscopic growth and CCN
81 activation of limonene-derived OS (with molecular weight of 250 Da) and their mixtures with
82 ammonium sulfate. Hygroscopicity of pure limonene-derived OS was weak, and its hygroscopic
83 growth factors were determined to be 1.0 at 80% RH and 1.2 at 93% RH (Hansen et al., 2015).
84 Estillore et al. (2016) investigated hygroscopic growth of a series of OS, including potassium salts
85 of glycolic acid sulfate, hydroxyacetone sulfate, 4-hydroxy-2,3-epoxybutane sulfate, and 2-
86 butenediol sulfate, as well as sodium salts of benzyl sulfate, methyl sulfate, ethyl sulfate, and
87 propyl sulfate. Continuous hygroscopic growth (i.e. without obvious deliquescence) was observed
88 for these OS aerosols (Estillore et al., 2016); in addition, their hygroscopic growth factors at 85%
89 RH were determined to vary between 1.29 and 1.50, suggesting that their hygroscopicity showed
90 substantial variation. In summary, it is fair to state that hygroscopic properties and CCN activities
91 of OS have not been well understood.
5 92 In this work, three complementary techniques were used to investigate hygroscopic properties
93 and CCN activities of a series of OS, including sodium methyl sulfate, sodium ethyl sulfate,
94 sodium octyl sulfate, sodium dodecyl sulfate, potassium hydroxyacetone sulfate, potassium 3-
95 hydroxy phenyl sulfate, potassium benzyl sulfate, potassium 2-methyl benzyl sulfate, potassium
96 3-methyl benzyl sulfate, potassium 2,4-dimethyl benzyl sulfate and potassium 3,5-dimethyl benzyl
97 sulfate. A vapor sorption analyzer was employed to measure mass change of these OS samples as
98 a function of RH. In addition, hygroscopic growth (change in mobility diameters) and CCN
99 activation of submicron aerosol particles were studied for sodium methyl sulfate, sodium ethyl
100 sulfate and sodium octyl sulfate, using a humidity tandem differential mobility analyzer (H-TDMA)
101 and a cloud condensation nuclei counter (CCNc). Due to their very limited quantities, we could
102 not carry out H-TDMA and CCNc measurements for other OS samples which were synthesized
103 by us. In addition, we also investigated the impacts of sodium methyl sulfate, sodium ethyl sulfate
104 and sodium octyl sulfate on hygroscopic properties and CCN activities of ammonium sulfate.
105 2 Experimental section
106 2.1 Chemicals and reagents
107 Sodium methyl sulfate (CH3SO4Na, >98%) and sodium ethyl sulfate (C2H5SO4Na, >98%)
108 were purchased from Tokyo Chemical Industry (TCI); sodium octyl sulfate (C8H17SO4Na, >99%),
109 sodium dodecyl sulfate (C12H25SO4Na, >99%) and ammonium sulfate (>99.5%) were supplied by
110 Aldrich. The other seven OS, including potassium hydroxyacetone sulfate, potassium 3-hydroxy
111 phenyl sulfate, potassium benzyl sulfate, potassium 2-methyl benzyl sulfate, potassium 3-methyl
112 benzyl sulfate, potassium 2,4-dimethyl benzyl sulfate and potassium 3,5-dimethyl benzyl sulfate,
113 were synthesized using the method described by Huang et al. (2018), and their purities were found
6 114 to be >95% using nuclear magnetic resonance analysis. Chemical formulas and molecular
115 structures of OS investigated in this study can be found in Figure 1.
116
117 Figure 1. Chemical formulas and molecular structures of organosulfates investigated in this study.
118
119 2.2 VSA experiments
120 A vapor sorption analyzer (VSA), commercialized by TA Instruments (New Castle, DE,
121 USA), was used to measure mass change of organosulfates as a function of RH. Experimental
7 122 details can be found in our previous studies (Chen et al., 2019; Gu et al., 2017; Guo et al., 2019;
123 Tang et al., 2019b), and are thus described here briefly. Experiments were conducted at 25±0.1 oC
124 and in the RH range of 0-90%. A high precision balance was used to measure the sample mass at
125 different RH with a stated sensitivity of <0.1 μg, and the dry mass of samples used in this work
126 was typically around 1.0 mg.
127
128 Figure 2. Change in RH (black curve, left y axis) and normalized sample mass (blue curve, right
o 129 y axis) of CH3SO4Na with of time in a typical vapor sorption analyzer experiment at 25 C.
130
131 Changes in RH and normalized mass of a CH3SO4Na sample with time are displayed in Figure
132 2 as an example to illustrate the VSA experimental procedure. As shown in Figure 2, the mass of
133 OS samples at different RH was determined by the VSA using the following method. RH was set
134 to <1% to dry the sample; after the sample mass was stabilized, RH was increased to 90% stepwise
135 with an interval of 10% per step; at the end, RH was changed back to <1% to dry the sample again.
136 The sample was considered to reach the equilibrium at a given RH when the mass change was
8 137 measured to be <0.1% within 30 min. All the experiments were conducted at least three times in
138 our work. The sample mass at a given RH (m) was normalized to that at <1% RH (m0) to determine
139 the mass growth factor, defined as m/m0.
140 2.3 H-TDMA experiments
141 A custom-built hygroscopicity tandem differential mobility analyzer (H-TDMA) was used to
142 measure the mobility diameters of OS aerosol particles at different RH (5-90%) at 24±1 oC. The
143 instrument was detailed elsewhere (Jing et al., 2016; Peng et al., 2016), and therefore only a brief
144 introduction is given here. A commercial atomizer (MSP 1500) was used to produced polydisperse
145 aerosol particles from dilute OS solutions in water (around 0.1 wt %), and the generated aerosol
146 was dried to <5% RH by passing the aerosol flow through a Nafion dryer (MD-110-12S) and then
147 a silica gel diffusion dryer. The dry aerosol flow was subsequently split to two flows. One aerosol
148 flow was sent to the vent, and the other aerosol flow (0.3 L min-1) was passed through the first
149 differential mobility analyzer (DMA) to produce quasi-monodisperse aerosol particles with a
150 mobility diameter of 100 nm. After that, the aerosol flow was humidified to a desired RH by
151 flowing through a humidification section, which was made of two Nafion tubes (MD-700-12F-1)
152 connected in series, and the residence time in the humidification section was ~27 s. Finally, the
153 size distribution of humidified aerosol was measured by a scanning mobility particle sizer (SMPS),
154 which consisted of the second DMA coupled to a condensation particle counter (CPC 3776, TSI).
155 RH of the aerosol flow and the sheath flow in the second DMA were maintained to be equal and
156 monitored using a commercial dew-point hygrometer (Michell, UK) with a stated uncertainty of
157 ±0.08% RH. In addition, the flow rate ratio of the sheath flow to the aerosol flow was set to 10:1
158 for both DMAs.
9 159 Hygroscopic growth factors (GF), defined as d/d0 (d is the mobility diameter at a given RH
160 and d0 is the mobility diameter at RH <5%) were reported. All the experiments were conducted in
161 triplicate. During our experiments, ammonium sulfate was used to calibrate the H-TDMA system
162 routinely, and the absolute differences between the measured and theoretical GF at 90% RH were
163 found to be within 0.04, confirming the robustness of our measurements.
164 2.4 CCN experiments
165 The CCN activity of aerosol particles was determined using a commercial cloud condensation
166 nuclei counter (CCNc, CCN-100, Droplet Measurement Technologies, Longmont, CO, USA)
167 described in previous studies (Roberts and Nenes, 2005; Lance et al., 2006; Moore et al., 2010).
168 Polydisperse aerosol particles were generated using a commercial atomizer (TSI 3076), in which
169 concentrations of solutions used were around 0.1 g/L. The wet aerosol flow generated was passed
170 through two silica gel diffusion dryers to reduce its RH to <5% RH. After that, a dry aerosol flow
171 (~800 mL min-1) was passed through a DMA (TSI 3081) in size scanning mode to produce quasi-
172 monodisperse aerosols, and subsequently the aerosol flow was split to two streams: one stream
173 (~300 mL/min) was sampled into a commercial CPC (TSI 3775) to measure total number
174 concentrations of aerosol particles ([CN]), and the second flow (~500 mL/min) was sampled into
175 the cloud condensation nuclei counter (CCNc, CCN-100) to measure number concentrations of
176 CCN ([CCN]).
177 Activation fractions ([CCN]/[CN]) of size-resolved dry particles were determined using the
178 Scanning Mobility CCN Analysis (SMCA) method described elsewhere (Moore et al., 2010). In
179 brief, the DMA was operated in the scanning voltage mode, and thus one activation curve
180 (activation fractions as a function of dry diameter) could be obtained in 60-120s. The multiple
181 charge effect was also corrected in this method, and in our work the supersaturation (SS) was set
10 182 in the range of 0.45-1.13% with the stated uncertainty to be ±0.01%. As shown in Figure 3,
183 activation fractions of sodium methyl sulfate (methyl-OS) and its internally mixed aerosol with
184 ammonium sulfate were measured at four different SS with dry mobility diameters between 20 and
185 100 nm. Activation fractions were fitted versus dry diameters, and the critical particle diameter
186 (d50) was determined as the dry diameter at which the activation fraction is equal to 0.5. During
187 our measurements, ammonium sulfate was used to calibrate supersaturations, and the Pitzer-ion
188 interaction model was applied in the calibration procedure to account for incomplete dissociation
189 of ammonium sulfate at droplet activation (Pitzer and Mayorga, 1973; Clegg and Brimblecombe,
190 1988). The corrected supersaturations were reported in our work.
191
192 Figure 3. Activation fractions of methyl-OS and its internally mixed aerosol particles with
193 ammonium sulfate (AS) as a function of dry particle diameter at four supersaturations.
194
11 195 3 Results and discussion
196 3.1 Mass growth of organosulfates
197 Figure 4 displays mass growth factors of sodium methyl sulfate (methyl-OS), sodium ethyl
198 sulfate (ethyl-OS), sodium octyl sulfate (octyl-OS), sodium dodecyl sulfate (dodecyl-OS) and
199 potassium hydroxyacetone sulfate, and the data are also listed in Table 1. Figure 4a suggests that
200 methyl-OS was deliquesced when RH was increased from 40% to 50%, and after that mass growth
201 factors increased further with RH. The mass of ethyl-OS was moderately increased (by ~11%)
202 when RH was increased from 40% to 50%, and further increase in RH to 60% led to additional
203 while small increase in sample mass (by ~2%); the increase in sample mass at 50% and 60% RH
204 may be because ethyl-OS were partially deliquesced at this stage (Ling and Chan, 2008; Mikhailov
205 et al., 2009). When RH was increased to 70%, ethyl-OS was completely deliquesced, and further
206 increase in RH (to 80% and 90%) resulted in further increase in sample mass. Octyl-OS was only
207 deliquesced when RH was increased from 80% to 90%, whereas no significant water uptake was
208 observed for dodecyl-OS even at 90% RH. The mass growth factors at 90% RH were determined
209 to be 3.65±0.06, 3.58±0.02 and 1.59±0.01 for methyl-OS, ethyl-OS, and octyl-OS, respectively.
210
12 211 Figure 4. Mass growth factors of (a) methyl-, ethyl- and octyl-OS and (b) dodecyl-OS and
212 potassium hydroxyacetone sulfate as a function of RH at 25 oC. Please note that error bars are
213 included, but they are too small to be clearly visible.
214
215 Mass growth factors of seven potassium organosulfates were also investigated, including
216 potassium hydroxyacetone sulfate, potassium 3-hydroxy phenyl sulfate, potassium benzyl sulfate,
217 potassium 2-methyl benzyl sulfate, potassium 3-methyl benzyl sulfate, potassium 2,4-dimethyl
218 benzyl sulfate and potassium 3,5-dimethyl benzyl sulfate. All the compounds did not show
219 measurable water uptake at 80% RH. When RH was increased to 90%, as shown in Figure 4b, a
220 significant increase in mass was observed for potassium hydroxyacetone sulfate particles,
221 suggesting the occurrence of deliquescence, and the mass growth factor was determined to be
222 2.20±0.03 at 90% RH. Furthermore, no significant water uptake was observed for the other six
223 potassium organosulfates even when RH was increased to 90%. We should mention that
224 occasionally small increase in sample mass (up to 10-20%) was observed for a few samples when
225 RH was increased from 80% to 90%, and such small increase in sample mass may be caused by
226 water uptake of impurities (such as potassium hydroxide) contained in these synthesized
227 compounds. When such increase sample mass was observed when RH was increased from 80% to
228 90%, we carried out at least four duplicate experiments, and all the other experiments confirmed
229 that mass increase was insignificant.
230
231 Table 1. Mass growth factors (m/m0) and water-to-solute ratios (WSRs) as a function of RH (10-
232 90 %) at 25 oC for sodium methyl sulfate, sodium ethyl sulfate, sodium octyl sulfate and potassium
233 hydroxyacetone sulfate. All the errors given in this work are standard deviations.
13 sodium methyl sulfate sodium ethyl sulfate RH (%) m/m0 WSR m/m0 WSR 10 1.00±0.01 - 1.00±0.01 - 20 1.00±0.01 - 0.99±0.01 - 30 0.99±0.01 - 0.99±0.01 - 40 1.00±0.02 - 1.02±0.03 - 50 1.36±0.02 2.68±0.04 1.13±0.01 - 60 1.55±0.03 4.06±0.09 1.15±0.01 - 70 1.83±0.05 6.16±0.17 1.79±0.02 6.46±0.07 80 2.31±0.04 9.73±0.18 2.27±0.02 10.48±0.11 90 3.65±0.06 19.75±0.34 3.58±0.02 21.19±0.14
sodium octyl sulfate potassium hydroxyacetone sulfate RH (%) m/m0 WSR m/m0 WSR 10 1.00±0.01 - 1.00±0.01 - 20 1.00±0.01 - 1.00±0.01 - 30 1.00±0.01 - 1.00±0.01 - 40 1.00±0.01 - 1.00±0.01 - 50 1.00±0.01 - 1.00±0.01 - 60 1.00±0.01 - 1.00±0.01 - 70 1.00±0.01 - 1.00±0.01 - 80 1.01±0.01 - 1.00±0.01 - 90 1.59±0.01 7.63±0.02 2.20±0.03 12.84±0.18 234
235 For deliquesced samples, measured mass changes can be converted to water to solute ratios
236 (WSRs), defined as the molar ratio of H2O to sulfur. The WSRs data are summarized in Table 1
237 for sodium methyl sulfate, sodium ethyl sulfate, sodium octyl sulfate and potassium
238 hydroxyacetone sulfate. As shown in Table 1, WSRs at 90% RH were determined to be 19.75±0.34,
239 21.19±0.14, 7.63±0.02 and 12.84±0.18 for sodium methyl sulfate, sodium ethyl sulfate, sodium
240 octyl sulfate and potassium hydroxyacetone sulfate at 25 oC.
14 241 3.2 Hygroscopic growth of aerosols
242 3.2.1 Organosulfates
243 H-TDMA was employed to measure hygroscopic growth factors of 100 nm methyl-, ethyl-
244 and octyl-OS aerosols as a function of RH (up to 90%), and the results are shown in Figure 5 and
245 Table 2. In addition, no significant hygroscopic growth was observed for dodecyl-OS for RH up
246 to 90%. We did not investigate hygroscopic growth of other OS aerosols due to the very small
247 quantity of these synthesized compounds.
248
249 Figure 5. Hygroscopic growth factors of (a) methyl- and octyl-OS and (b) ethyl-OS aerosols as a
250 function of RH. Solid curves represent fitted curves in our work using Eq. (1). For comparison,
251 the fitted curves reported by Estillore et al. (2016) are presented by dashed curves.
252
253 As shown in Figure 5, methyl-, ethyl- and octyl-OS aerosols all exhibited continuous
254 hygroscopic growth without obvious phase transitions. To our knowledge, only one previous study
15 255 (Estillore et al., 2016) investigated hygroscopic growth of methyl- and ethyl-OS aerosols using a
256 H-TDMA, and continuous hygroscopic growth was also observed. The continuous growth
257 behavior can be attributed to the amorphous state of aerosol particles, which would take up water
258 at very low RH. For methyl-OS aerosol, GFs were determined in our work to be 1.53±0.01,
259 1.63±0.01 and 1.83±0.03 at 80%, 85% and 90% RH; for comparison, its GF was measured to be
260 1.50 at 85% RH by Estillore et al. (2016), only ~8% smaller than our result. In our work, GFs were
261 determined to be 1.47±0.01, 1.60±0.02 and 1.79±0.02 at 80%, 85% and 90% RH for ethyl-OS
262 aerosol; for comparison, it was measured to be 1.45 at 85% RH in the previous study (Estillore et
263 al., 2016), only ~9% smaller than our result. As DMA sizing typically has a relative uncertainty of
264 5-7% (Wiedensohler et al., 2012), our measured GFs agree very well with those reported by
265 Estillore et al. (2016) for methyl- and ethyl-OS, while the highest RH we reached was 90%,
266 compared to 85% by Estillore et al. (2016). With respect to octyl-OS aerosol, GF were determined
267 to be 1.11±0.02, 1.17±0.01 and 1.21±0.02 at 80%, 85% and 90% RH in our work; to our knowledge,
268 hygroscopic growth of octyl-OS aerosol has not been explored previously. Compared to
269 ammonium sulfate (1.75 at 90% RH), GFs at 90% RHs were found to be slightly larger for methyl-
270 and ethyl-OS, but significantly smaller for octyl-OS.
271
272 Table 2. Hygroscopic growth factors (GFs) of methyl-, ethyl- and octyl-OS aerosols at different 273 RH. All the errors given in this work are standard deviations.
RH (%) sodium methyl sulfate sodium ethyl sulfate sodium octyl sulfate
5 1.00±0.01 1.00±0.01 1.00±0.01
10 1.01±0.01 1.00±0.01 1.00±0.01
20 1.04±0.01 1.03±0.01 0.99±0.01
30 1.08±0.01 1.04±0.01 1.00±0.01
16 40 1.14±0.01 1.08±0.01 0.99±0.01
50 1.22±0.01 1.15±0.01 0.99±0.01
60 1.29±0.01 1.25±0.01 1.01±0.01
70 1.39±0.01 1.34±0.01 1.05±0.01
75 1.44±0.01 1.39±0.01 1.07±0.01
80 1.53±0.01 1.47±0.01 1.11±0.02
85 1.63±0.01 1.60±0.02 1.17±0.01
90 1.83±0.03 1.79±0.02 1.21±0.02
274
275 When aerosol particles take up water continuously, the RH-dependent GFs can usually be
276 fitted using Eq. (1) (Kreidenweis et al., 2005):
RH RH RH 277 GF = [1 + (푎 + 푏 ∙ + 푐 ∙ ( )2) ∙ ]1/3 (1) 100 100 100−RH
278 where a, b and c are coefficients obtained from fitting using Eq. (1). As shown in Figure 5,
279 hygroscopic growth factors of methyl-, ethyl- and octyl-OS aerosols can be fitted by Eq. (1), and
280 the obtained coefficients (a, b and c) are summarized in Table 3.
281
282 Table 3. The three coefficients (a, b and c) obtained by using Eq. (1) to fit RH-dependent GFs for
283 sodium methyl sulfate, sodium ethyl sulfate and sodium octyl sulfate aerosols.
organosulfates a b c
sodium methyl sulfate 0.42182 1.20336 -1.15508
sodium ethyl sulfate 0.00174 1.61805 -1.15502
sodium octyl sulfate -0.31868 0.86233 -0.44623
284
17 285 3.2.2 Comparison between VSA and H-TDMA measurements
286 Figure 4 shows that obvious deliquescence transitions were observed for methyl-, ethyl-, and
287 octyl-OS in the VSA experiments, because samples used in VSA experiments may be crystalline
288 salts; in contrast, as revealed by Figure 5, continuous hygroscopic growth without obvious phase
289 transitions was observed for methyl-, ethyl- and octyl-OS aerosol particles in H-TDMA
290 measurements, suggesting that these aerosol particles which were produced by drying aqueous
291 droplets to <5% RH may exist in amorphous state. Estillore et al. (2016) employed a H-TDMA to
292 investigate hygroscopic properties of several OS aerosols, and similarly they found that those
293 aerosols, including methyl-OS, ethyl-OS and potassium hydroxyacetone sulfate which were also
294 examined in our work, displayed continuous hygroscopic growth.
295 For completely deliquesced particles, if it is assumed that the particle is spherical and that the
296 particle volume at a given RH is equal to the sum of the dry particle volume and the volume of
297 particulate water, particle mass change, measured using the VSA, can then be converted to
298 hygroscopic GF, using Eq. (2):
3 푚 𝜌 299 GF = √1 + ( − 1) ∙ 0 (2) 푚0 𝜌푤
300 where ρ0 and ρw are the density of the dry sample and water, respectively. The density of methyl-,
301 ethyl- and octyl-OS particles were reported to be 1.60, 1.46 and 1.19 g cm-3 with an uncertainty of
302 20-30% (Kwong et al., 2018; ChemistryDashboard, 2021). Figure 6 compares VSA-derived GFs
303 and those measured using H-TDMA for methyl-, ethyl- and octyl-OS, and it can be concluded that
304 for RH at which samples used in the VSA experiments were deliquesced, GFs derived from mass
305 change measured using VSA agree relatively well with those directly measured using H-TDMA.
306 For example, at 90% RH GFs were measured by H-TDMA to be 1.83±0.03, 1.79±0.02 and
307 1.21±0.02 for methyl-, ethyl- and octyl-OS, while at the same RH their GFs derived from VSA
18 308 measurements were found to be 1.74±0.01, 1.68±0.01 and 1.19±0.01, only 6% (or less) smaller
309 than those measured using H-TDMA. The small but systematical differences between VSA and
310 H-TDMA results, as evident from Figure 6, could stem from volume additivity assumption used
311 to convert mass growth to diameter growth, uncertainties in OS densities, and DMA sizing errors.
312 313 Figure 6. Comparison between hygroscopic GFs of methyl-, ethyl- and octyl-OS derived from
314 VSA experiments to those measured using H-TDMA. Please note that H-TDMA results are
315 presented as the three-parameter curves obtained. Error bars are included, but they are too small
316 to be clearly visible.
317
318 3.2.3 Internally mixed aerosols
319 We also investigated hygroscopic properties of methyl-, ethyl- and octyl-OS aerosols
320 internally mixed with ammonium sulfate (AS), and the results are summarized in Table 4. Figure
321 7a displays GFs of 100 nm methyl-OS/AS mixed aerosols with mass ratios of 1:1 and 1:5. The 1:1
19 322 mixed aerosol particle showed a deliquescence transition at 70% RH, while the 1:5 mixed aerosols
323 showed a deliquescence transition at 75% RH, which was lower than the deliquescence RH (DRH,
324 80%) of AS. Here the DRH is defined as the RH at which the mixed aerosols are completely
325 deliquesced (Choi and Chan, 2002). Figure 7a suggests that before full deliquescence, significant
326 hygroscopic growth was also observed, i.e. pre-deliquescence of mixed particles occurred when
327 RH was lower than their DRH. Pre-deliquescence was widely reported in previous studies which
328 investigated hygroscopic properties of inorganic/organic mixed aerosols (Choi and Chan, 2002;
329 Prenni, 2003; Wise et al., 2003; Brooks, 2004; Marcolli and Krieger, 2006; Wu et al., 2011; Lei et
330 al., 2014; Jing et al., 2016; Estillore et al., 2017). For example, Choi and Chan. (2002) investigated
331 hygroscopic behaviors of internal mixed particles which consisted of water-soluble organic
332 compounds and AS, and found that the internal mixing with organics (such as malonic and citric
333 acids) could reduce the DRH of AS, due to the ability of organics to absorb water at low RH.
334 Internal mixing with ethyl- and octyl-OS could also reduce the DRH of AS. As shown in
335 Figure 7b, ethyl-OS/AS mixed aerosols were deliquesced at 70% RH when the mass ratio of ethyl-
336 OS to AS was 1:1 and at 80% RH when the mass ratio was 1:5. In addition, Figure 7c suggested
337 that the deliquescence of octyl-OS/AS aerosols took place at 75% RH for the 1:1 mixture and at
338 80% for 1:5 mixture.
20 339
340 Figure 7. Hygroscopic growth factors of (a) methyl-OS/AS, (b) ethyl-OS/AS, and (c) octyl-OS/AS
341 aerosols as a function of RH. The mass ratios of methyl-, ethyl-, and octyl-OS to AS were 1:1 and
342 1:5, respectively. Solid curves represent hygroscopic growth factors of mixed aerosols predicted
343 using the ZSR method.
344
345 Table 4. Hygroscopic GF of methyl-, ethyl, and octyl-OS internally mixed with AS (their mass 346 ratios are 1:1 and 1:5) at different RH. All the errors given in this work are standard deviations.
RH methyl-OS/AS ethyl-OS/AS octyl-OS/AS (%) 1:1 1:5 1:1 1:5 1:1 1:5 5 1.00±0.01 1.00±0.01 1.00±0.01 1.00±0.01 1.00±0.01 1.00±0.01 10 1.00±0.01 1.00±0.01 1.00±0.01 1.00±0.01 1.00±0.01 1.00±0.01
21 20 1.01±0.01 1.00±0.01 1.00±0.01 1.00±0.01 1.00±0.01 1.00±0.01 30 1.03±0.01 1.00±0.01 1.00±0.01 1.00±0.01 1.01±0.01 1.01±0.01 40 1.06±0.01 1.00±0.01 1.02±0.01 0.99±0.01 1.01±0.01 1.01±0.01 50 1.10±0.01 1.02±0.01 1.07±0.01 0.99±0.01 1.01±0.01 1.01±0.01 60 1.20±0.02 1.06±0.01 1.13±0.01 1.02±0.01 1.02±0.01 1.01±0.01 70 1.38±0.01 1.19±0.01 1.33±0.01 1.13±0.03 1.07±0.01 1.05±0.01 75 1.43±0.01 1.40±0.01 1.38±0.01 1.34±0.04 1.23±0.01 1.14±0.03 80 1.52±0.01 1.48±0.01 1.46±0.01 1.45±0.02 1.28±0.01 1.40±0.02 85 1.63±0.01 1.59±0.01 1.56±0.01 1.54±0.02 1.35±0.02 1.51±0.02 90 1.79±0.01 1.74±0.02 1.74±0.02 1.72±0.02 1.47±0.02 1.69±0.04 347
348 The Zdanovskii-Stokes-Robinson (ZSR) method (Stokes and Robinson, 1966) has been
349 widely used to predict hygroscopic growth of internally mixed aerosol particles, assuming that the
350 interaction among individual species are negligible and that individual species in the mixed
351 particles take up water independently. According to the ZSR method, GF of a mixed particle, GFmix,
352 can be calculated using Eq. (3) (Malm and Kreidenweis, 1997):
3 3 353 GFmix = √∑(휀푖 ∙ GF푖 ) (3)
354 where GFi is the GF of ith species the dry mixed particle contains. The volume fraction of the ith
355 species in the dry mixed particle, εi, can be calculated using Eq. (4):
푚푖/𝜌푖 356 휀푖 = (4) ∑(푚푖/𝜌푖)
357 where mi and ρi are the mass fraction and density of the ith species. GFs of pure OS, measured in
358 our work using H-TDMA and presented in Section 3.2.1, and GFs of AS, calculated using the E-
359 AIM model (Clegg et al., 1998; Wexler and Clegg, 2002), were used as input to predict GFs of
360 methyl-, ethyl- and octyl-OS internally mixed with AS. Comparisons between measured and
361 predicted GFs are displayed in Figure 7 for OS/AS mixed aerosols.
22 362 As shown in Figure 7a, GFs of methyl-OS/AS mixed aerosols (both the 1:1 and 1:5 mixtures)
363 could be well predicted using the ZSR method when RH was <60% or >80%, while the ZSR
364 method underestimated their GFs at 70% and 75% RH. Such underestimation at 70% and 75% RH
365 is likely to due to that inorganic compounds (AS, in our work) may dissolve partially in the
366 organics/water solution (which can be formed at much lower RH due to continuous water uptake
367 of organics) before the mixed particle is completely deliquesced (Svenningsson et al., 2006;
368 Zardini et al., 2008; Wu et al., 2011); in contrast, the ZSR method assumes that individual species
369 take up water independently. As shown in Figure 7, the ZSR method also underestimated GFs at
370 70 and 75% RH for ethyl-OS/AS and octyl-OS/AS mixed aerosols, though good agreement
371 between measurement and prediction was found at other RH. For example, the ratios of partially
372 dissolved AS to total AS at 70% RH were estimated to be 0.95, 0.85 and 0.49 for methyl-OS/AS,
373 ethyl-OS/AS, and octyl-OS/AS mixtures with a mass ratio of 1:1.
374 3.3 Cloud condensation nucleation activities
375 Figures 3, S2 and S3 show CCN activation curves obtained at four supersaturations for
376 methyl-, ethyl- and octyl-OS aerosols and their internal mixtures with ammonium sulfate. Each
377 activation curve was fitted using a Boltzmann sigmoid function to derive the corresponding critical
378 particle diameter (d50), which was then used to calculate κccn using Eqs. (5a-5b) (Petters and
379 Kreidenweis, 2007):
4퐴3 380 휅ccn = 3 2 (5a) 27푑50 ln 푆푐
4𝜎푠/푎푀푤 381 퐴 = (5b) 푅푇𝜌푤
382 where Sc is the critical saturation ratio (1+SS) of water; d50 is the critical particle diameter; A is a
383 constant which describes the Kelvin effect on a curved surface of a droplet, and depends on the
384 surface tension (σs/a), molecular weight (Mw), density (ρw) of water, temperature (T) and the
23 385 universal gas constant (R). Table 5 summarizes critical diameters at different supersaturations for
386 aerosol particles examined in this work and their κccn values.
387
388 Table 5. Single hygroscopicity parameters derived from hygroscopic growth (κgf) and CCN
389 activity measurements (κccn) for methyl-, ethyl- and octyl-OS and their internal mixtures with
390 ammonium sulfate (AS). All errors given were standard deviations.
aerosol mass ratio SS (%) d50 (nm) κccn average κccn κgf methyl-OS - 0.45 52.9±0.9 0.432-0.477 0.459±0.021 0.537-0.604 - 0.67 41.1±0.8 0.416-0.468 - 0.87 33.3±0.4 0.471-0.507 - 1.13 28.8±0.5 0.431-0.477 methyl-OS/AS 1:5 0.45 51.9±0.5 0.467-0.492 0.453±0.027 0.454-0.495 1:5 0.67 41.6±0.4 0.411-0.436 1:5 0.87 33.7±0.5 0.453-0.490 1:5 1.13 29.2±0.6 0.412-0.464 ethyl-OS - 0.45 55.5±0.8 0.375-0.410 0.397±0.010 0.505-0.548 - 0.67 42.8±0.6 0.376-0.406 - 0.87 35.3±0.5 0.395-0.428 - 1.13 30.2±0.3 0.382-0.408 ethyl-OS/AS 1:5 0.45 52.3±1.2 0.437-0.504 0.458±0.024 0.435-0.474 1:5 0.67 41.0±0.5 0.426-0.459 1:5 0.87 33.4±0.6 0.463-0.512 1:5 1.13 29.2±0.6 0.409-0.462 octyl-OS - 0.45 70.0±1.2 0.186-0.207 0.206±0.008 0.076-0.096 - 0.67 53.2±0.6 0.196-0.211 - 0.87 44.1±0.7 0.202-0.221 - 1.13 37.1±0.8 0.200-0.227 octyl-OS/AS 1:5 0.45 53.7±0.9 0.413-0.456 0.436±0.009 0.388-0.464 1:5 0.67 41.1±0.5 0.426-0.458 1:5 0.87 34.4±0.5 0.427-0.462
24 1:5 1.13 29.5±0.2 0.413-0.434 391
392 As shown in Table 5, κccn values were determined to be 0.459±0.021, 0.397±0.010 and
393 0.206±0.008 for methyl-, ethyl- and octyl-OS, decreasing with alkyl chain length, and this suggests
394 that the addition of hydrophobic hydrocarbon functional groups to OS reduced their hygroscopicity.
395 Decrease in hygroscopicity of OS compounds with the increase in the number of carbon atoms
396 was also observed under subsaturated conditions (Section 3.2). In addition, we investigated CCN
397 activities of alkyl-OS/AS mixed aerosols with a mass ratio of 1:5, and κccn values were determined
398 to be 0.453±0.027, 0.458±0.024 and 0.436±0.009 for methyl-OS/AS, ethyl-OS/AS and octyl-
399 OS/AS.
400 3.3.1 Comparison between H-TDMA and CCN activities measurements
401 It is suggested that the single hygroscopicity parameter, κ, could describe aerosol-water
402 interactions under both sub- and supersaturated conditions (Petters and Kreidenweis, 2007). The κ
403 values derived from CCN activity measurements, κccn, have been illustrated above; the κ values
404 derived from H-TDMA measurements, κgf, can be calculated using Eq. (6) (Petters and
405 Kreidenweis, 2007; Tang et al., 2016):
1−RH 406 휅 = (GF3 − 1) (6) gf RH
407 In this work GF measured at 90% RH were used to calculate κgf values, which are also listed in
408 Table 5. Eq. (6) does not take into account the Kelvin effect as the effect is small for 100 nm
409 particles (Tang et al., 2016).
410 Figure 8 compares κccn and κgf values for the six types of aerosol particles examined. For pure
411 OS, κccn of methyl-OS (0.459±0.021) and ethyl-OS (0.397±0.010) were smaller than their κgf
412 values (0.537-0.604 and 0.505-0.548), but the relative differences do not exceed 25%. Such a
25 413 difference (<25%) may not be significant if all the uncertainties associated with deriving κ from
414 measured hygroscopic growth and CCN activities (Petters and Kreidenweis, 2007). Octyl-OS
415 appears to be an exception, and the average κccn (0.206) was ~2.4 times larger than the average κgf
416 (0.086). In addition, no significant difference was observed between κccn and κgf for all the alkyl-
417 OS/AS mixed aerosols.
418
419 Figure 8. Comparison of κ values derived from hygroscopic growth (κgf) with these derived from
420 CCN activities (κccn) for methyl-, ethyl- and octyl-OS aerosols as well as their internal mixtures
421 with ammonium sulfate (the mass ratio was 1:5).
422
423 Significant differences between κgf and κccn were reported in previous studies (Petters et al.,
424 2009; Wex et al., 2009; Hansen et al., 2015), attributed to several factors discussed below. Petters
425 and Kreidenweis. (2008) demonstrated that cloud droplet activation was highly sensitive to the
426 solubility for sparingly soluble compounds in the range of 5×10-4-2×10-1, expressed as volume of
26 427 solute per unit volume of water (Petters and Kreidenweis, 2008). Compared to the highly soluble
428 methyl- and ethyl-OS (their solubilities are 0.127-0.219 and 0.075-0.151), the solubility of octyl-
429 OS (8.43×10-4-4.26×10-2) (Chemistry Dashboard, 2021) is rather limited, and incomplete
430 dissolution at subsaturated condition in H-TDMA measurements may lead to underestimation of
431 κgf values for octyl-OS; as a result, the solubility limit may explain the observed difference between
432 κgf and κccn for octyl-OS. Furthermore, surface tension is a key factor to influence critical
433 supersaturations at which aerosol particles are activated to cloud droplets (Petters and Kreidenweis,
434 2013). We measured surface tensions of alkyl-OS and alkyl-OS/AS (the mass ratio was 1:5)
435 solutions, and the results are shown in Table S1 and Figure S4. The surface tension of octyl-OS is
436 much lower than that of pure water, leading to significant reduction in critical supersaturations and
437 thus overestimation of its κccn value; for comparison, the surface tension depression is also visible
438 but much less pronounced for octyl-OS/AS mixed aerosol. Overall, we proposed that solubility
439 limit and surface tension reduction may both contribute to the observed discrepancy between κgf
440 and κccn values for octyl-OS aerosol. We note that some numerical models (Petters and
441 Kreidenweis, 2008, 2013; Riipinen et al., 2015) are available to quantitatively assess contribution
442 of solubility limit and surface tension reduction to the discrepancy between κgf and κccn.
443 4. Conclusions
444 Organosulfates (OS) may contribute significantly to secondary organic aerosols in various
445 locations over the globe; however, their hygroscopic properties and CCN activities have not been
446 well understood. In this work, three complementary techniques, including a vapor sorption
447 analyzer (VSA), a hygroscopicity tandem differential mobility analyzer (H-TDMA) and a cloud
448 condensation nuclei counter (CCNc), were employed to investigate interactions of several OS with
27 449 water vapor under sub- and supersaturated conditions, trying to get a comprehensive picture of
450 their hygroscopic properties and CCN activities.
451 VSA was used to measure mass change of OS samples with RH (0-90%). Obvious
452 deliquescence was found for sodium methyl sulfate (methyl-OS), sodium ethyl sulfate (ethyl-OS),
453 sodium octyl sulfate (octyl-OS) and potassium hydroxyacetone sulfate, and their mass growth
454 factors at 90% RH were determined to be 3.65±0.06, 3.58±0.02, 1.59±0.01 and 2.20±0.03,
455 respectively. No significant water uptake were observed up to 90% RH for other OS compounds
456 examined, including sodium dodecyl sulfate, potassium 3-hydroxy phenyl sulfate, potassium
457 benzyl sulfate, potassium 2-methyl benzyl sulfate, potassium 3-methyl benzyl sulfate, potassium
458 2,4-dimethyl benzyl sulfate and potassium 3,5-dimethyl benzyl sulfate. Hygroscopic properties of
459 methyl-, ethyl- and octyl-OS aerosols were also studied using H-TDMA, which measured mobility
460 diameters of aerosol particles as a function of RH. Continuous hygroscopic growth was observed
461 for methyl-, ethyl- and octyl-OS aerosols, and their growth factors at 90% RH were measured to
462 be 1.83±0.03, 1.79±0.02 and 1.21±0.02.
463 We further investigated CCN activities of methyl-, ethyl- and octyl-OS aerosols, and their
464 single hygroscopicity parameters, κccn, were determined to be 0.459±0.021, 0.397±0.010 and
465 0.206±0.008, respectively. For methyl- and ethyl-OS aerosols, single hygroscopicity parameters
466 derived from CCN activities (κccn) agree reasonably well with those derived from H-TDMA
467 measurements (κgf) with relative differences being <25%. However, κccn was found to be ~2.4
468 times larger than κgf for octyl-OS, and we show that solubility limit and surface tension reduction
469 may both contribute to such discrepancy observed.
470
471 Data availability. Data used in this paper can be found in the main text or supplement.
28 472 Competing interests. The authors declare that they have no conflict of interest.
473 Author contribution. Mingjin Tang conceived this work; Ru-Jin Huang, Yuqing Zhang, Xiang
474 Ding and Xinming Wang chose and provided samples investigated in this work; Chao Peng,
475 Lanxiadi Chen, Yuqing Zhang and Xiang Ding conducted VSA measurements; Chao Peng,
476 Weigang Wang and Maofa Ge conducted H-TDMA measurements; Patricia N. Razafindrambinina,
477 Kotiba A. Malek and Akua A. Asa-Awuku conducted CCN activity measurements; Chao Peng,
478 Patricia N. Razafindrambinina, Kotiba A. Malek, Akua A. Asa-Awuku and Mingjin Tang analyzed
479 the data and prepared the manuscript with contribution from all the other coauthors.
480 Financial support
481 This work was funded by National Natural Science Foundation of China (91744204), China
482 Postdoctoral Science Foundation (2020M682931), Ministry of Science and Technology of China
483 (2018YFC0213901), Chinese Academy of Sciences international collaborative project
484 (132744KYSB20160036), State Key Laboratory of Loess and Quaternary Geology
485 (SKLLQG1921), Guangdong Foundation for Program of Science and Technology Research
486 (2019B121205006 and 2020B1212060053), Guangdong Science and Technology Department
487 (2017GC010501) and CAS Pioneer Hundred Talents program.
488
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